In a conventional copying machine or printer of an electrophotographic type, in general, a surface of a photosensitive member is electrically charged uniformly by a charging means and the uniformly charged surface of the photosensitive member is exposed to laser light, LED light or the like by an exposure means, so that an electrostatic latent image is written (formed) on the surface of the photosensitive member. After the electrostatic latent image is formed, the electrostatic latent image is developed by a developing means, so that an image is formed on the photosensitive member surface.
On the other hand, in order to downsize and simplify a constitution of the image forming apparatus, a plurality of examples of such an image forming apparatus that a plurality of recording electrodes arranged in a direction (scanning direction) perpendicular to a recording material conveyance direction are used to effect the development simultaneously with the latent image formation are disclosed.
For example, as represented by Japanese Patent Publication (JP-B) Hei 3-8544, in a multi-stylus printer using needle-like electrodes, an image forming electrode provided with a large number of needle-like electrodes and a cylindrical opposite electrode are oppositely disposed with a predetermined spacing (gap) in which a recording material is interposed in contact with the image forming electrode. In this state, a voltage corresponding to an image signal is applied to the image forming electrode to cause gap electric discharge, so that a toner image is formed.
Further, in a method as represented by Japanese Laid-Open Patent Application (JP-A) 2003-103824, first, by a plurality of writing electrodes contacting an inner surface of an image carrying member, electric charges are provided in a writing layer located at the inner surface of the image carrying member. Then, an electroconductive developing roller functions as the opposite electrode, so that an electrostatic latent image is formed on an electroconductive layer at the surface of the image carrying member. Simultaneously with the latent image formation, the latent image is developed with a developer on the developing roller, so that an image is formed.
In the above-described methods in which the development is effected simultaneously with the latent image formation by using the recording electrodes, a resolution of the image forming apparatus with respect to the scanning direction is determined by an interval between the plurality of divided recording electrodes. For example, in order to obtain the resolution of 600 dpi, the electrode interval is 42.3 μm. In order to realize a higher resolution, there is a need to decrease the electrode interval but there is a limit to realization of high density of the electrodes by fine processing technology. Therefore, there is a limit to realization of high resolution of the image by relying on only the decrease of the electrode interval.
On the other hand, in the image forming apparatus of the conventional type in which the electrostatic latent image is written on the surface of the photosensitive member by using the exposure means with the laser light or the LED light, in many cases, the image formation with the higher resolution is effected by modulation of lighting pulse or adjustment of light quantity. That is, width or length of the image for one pixel unit (dot) with respect to a main scan direction or a sub-scan direction is changed. These control operations are used in the case where serrated feeling represented in a stepwise shape when the image constituted by oblique lines or curved lines or the like is drawn or in the case where a not more than the resolution is intended to be faithfully reproduced. There are the case where the image forming apparatus automatically discriminates image information to perform the operations and the case where a user selects a high-resolution mode or the like to perform the operations.
The method of forming the image by using the above-described recording electrodes will be considered. When the image width for one pixel is tried to be adjusted by the control of a voltage applied to the electrodes, in the case of the multi-stylus type in which the electric charges are provided by the gap electric discharge, a charging property by the electric discharge is not uniform due to variation of the gap in the first place.
Further, in the method in which the electrostatic latent image is formed on the image carrying member surface as described in JP-A 2003-103824, a charge amount provided by adjusting the applied voltage is different, so that an amount of the developer subjected to the development is also changed. Therefore, reproducibility of the thin line or minute dot is poor and thus it is difficult to control clear image (dot) formation.
Accordingly, in an image forming process in which the development is effected simultaneously with the latent image formation by using the recording electrodes, the method of adjusting the image (dot) width for one pixel corresponding to the electrode interval has not been disclosed as yet.